The present invention relates to a stable current source that can be implemented as an integrated circuit without off-chip frequency or resistive references, and without precision on-chip resistors.
Stable integrated circuit current sources of the prior art often use a reference resistor. This reference resistor can be an external precision resistor or alternatively an internal, on-chip resistor. External resistors require an additional pin for the off-chip reference resistor, while on-chip resistors generally vary significantly due to process variations. To achieve a stable on-chip resistor, the resistor can be trimmed at the expense of testing and trimming costs. U.S. Pat. No. 4,680,535 discloses a stable current source that uses a reference precision external resistor, and U.S. Pat. No. 4,830,976 discloses an on-chip resistor that is held within desired tolerances by precision doping.
FIG. 7 illustrates another prior-art current source that uses a precision resistor R. The current through the resistor R is equal to Ibias=(VDDxe2x88x92VT)/R. This current can be used to control other parameters. As an example, in FIG. 7 the resulting bias signal Nbias is mirrored to Pbias. These bias signals are applied to a voltage controlled oscillator, which in this example is a five-stage ring oscillator fabricated from FET""s. Because the resistor R provides a known resistance, the voltage controlled oscillator provides a stable operating frequency. The signals Nbias and Pbias can also be mirrored to generate stable output currents referenced to VDD or to ground, as shown in FIG. 7.
U.S. Pat. No. 5,694,032 discloses a fourth approach, in which an accurate frequency source controls the switching rate of a switched capacitor network.
U.S. Pat. No. 5,694,033 relates to a fifth approach, in which diodes and FET""s are used to generate a current reference.
In spite of this earlier work, a need presently exists for an improved current source that provides a stable output current without external resistors, precision on-chip resistors, or precision frequency sources.
The preferred embodiment illustrated in FIG. 1 operates as a stable current source without requiring an external resistor, an internal precision resistor, or a precision frequency source. This circuit uses a switched capacitor circuit that includes the gate capacitor of a transistor. This gate capacitor is one of the best controlled layers in a CMOS process. Furthermore, the resistance of the switched capacitor circuit is inversely proportional to capacitance, making it possible to achieve a very large resistance value in a small area.
In the embodiment of FIG. 1, the bias current Ibias controls the frequency of oscillation of a voltage controlled oscillator, and the oscillating output signal generated by this oscillator controls the switching rate of the switched capacitor circuit.
The foregoing paragraphs have been provided by way of general introduction, and they should not be used to narrow the scope of the following claims.